An electrostatic actuator is an electromechanical device having a static electrode and a movable electrode, for exerting a controllable movement of the movable electrode relative to the static electrode, upon applying a driving voltage between the static and the movable electrodes. In actuators for exerting angular movement, the static and the movable electrodes are commonly called “stator” and “rotor”, respectively.
Perhaps the simplest electrostatic actuator is a pair of planar parallel plates, one being the stator and the other being the rotor. As the actuating voltage is applied, the plates attract each other generating an actuating force. The actuating force causes the rotor plate to tilt, bringing it closer to the stator plate. Although simple, a parallel-plate electrostatic actuator suffers from drawbacks, such as a non-linear dependence of the actuating force on the driving voltage, and the generation of a relatively small actuating force. Another well-known type of an electrostatic actuator is a so-called “comb” actuator. In a comb actuator, the rotor and the stator are made in the form of combs, or stacks, comprising planar parallel plates separated by a distance that is larger than thickness of the plates. When the actuating voltage is applied to the rotor and the stator combs, they attract each other generating a stronger actuating force than the plates of a two-plate actuator.
A micro-electromechanical system (MEMS) is a micro-sized mechanical structure having electrical circuitry fabricated together with the device by using microfabrication processes mostly derived from integrated circuit fabrication processes. The developments in the field of MEMS process engineering enabled batch production of electrostatic MEMS comb actuators that can be used in visual displays, optical attenuators and switches, and other devices. When a MEMS device is actuated, a micromirror supported by the MEMS device is tilted about a working axis, which makes an optical beam falling thereupon to steer from one output optical port to another, thereby realizing the switching function. By having a plurality of output ports disposed along a single line, a multiport optical switch can be constructed.
Referring to FIGS. 1A and 1B, a prior-art MEMS comb actuator 10 is shown in plan and side views, having a flexibly suspended rotor 11 with rotor plates 12 and a stator 13 with stator plates 14. The rotor and the stator plates 12 and 14 are “interdigitated”, that is, they are brought together such that the rotor plates 12 and the stator plates 14 are parallel to each other and overlap laterally without contacting each other, as shown in FIGS. 1A and 1B. A tilt axis 15 of the rotor 11 is perpendicular to the planes of the rotor and the stator plates 12 and 14. When a voltage is applied between the rotor plates 12 and the stator plates 14, the rotor and the stator plates 12 and 14 attract each other so as to increase the lateral overlap of the rotor 11 and the stator 13, by tilting the rotor 11 about the axis 15 in a direction of an arrow 19. Since the tilt axis 15 is perpendicular to the planes of the rotor and the stator plates 12 and 14, the plates 12 and 14 remain generally parallel during rotation of the rotor 11. As a result, almost linear actuation at a relatively high actuating force is produced by the MEMS comb actuator 10, sufficient for tilting a MEMS micromirror 11A supported by the rotor 11.
Unfortunately, the comb actuator 10, when used in a micromirror array application, has a serious drawback that outweighs its general advantage of a large force and linear actuation mentioned above. To perform as described above, the rotor 11 and the stator 13 of the MEMS comb actuator 10 have to be precisely aligned with respect to each other, with a precision of a few microns or better. Even a slight misalignment results in the rotor and the stator plates 12 and 14 attracting to each other laterally as shown by arrows 16 in FIG. 1A, which can cause turning of the rotor 11 about an axis 17 in a direction shown by an arrow 18. When the rotor 11 turns about the axis 17, the MEMS micromirror 11A supported by the rotor 11 turns in its own plane about the same axis 17 and collides with a neighboring micromirror, not shown, of the MEMS micromirror array, causing a catastrophic failure of the MEMS micromirror array.
The prior art is lacking a simple and efficient electrostatic comb actuator having suppressed in-plane rotation of a flexibly suspended rotor. Accordingly, it is a goal of the present invention to provide a comb actuator having lessened in-plane rotation of the rotor, and/or lessened sensitivity to rotor-stator misalignment, without a considerable reduction of magnitude of the electrostatic force produced by the actuator.